I would like to update a field in salesforce when someone opens an email sent from marketing cloud. I tried to add an engagement split in journey builder but i needed to add a wait time in. Does this mean that it will only asses the open of the email once? If I want something to happen only once the email is opened (in this case notify the sales person in Salesforce) I wondered if this is something that other users have done and how they have done this?
This isn't possible in journey builder as a journey represents the exact path a subscriber goes and isn't suitable for recurring tasks. To accomplish what you like to do you could create a scheduled automation in automation studio that queries the open tracking data view and updates the salesforce records accordingly.
The activities you need for this task are: